1. Field of the Invention
The present invention relates to the field of telecommunications. More particularly, the present invention relates to a system and a method for providing a free-space optical communication link.
2. Description of the Related Art
In a free-space optical communication link, link loss (path loss) variation of the communication link is significantly greater than the link loss of a fiber optic communication link. Additionally, the optical input power applied to an optical detector of a free-space optical communication link varies over a wide range depending on weather conditions between the transmitter and the receiver. Consequently, in longer distance links, the output of the optical transmitter for a conventional optical communication link is kept relatively high for compensating for link loss caused by relatively poor weather conditions.
Avalanche photo diode (APD) detectors are typically used for high-sensitivity and high-speed applications because the dynamic range of the optical input at the optical detector for such applications is large. Optical receivers using avalanche photo diode (APD) detectors, such as the model 54R receiver manufactured by BCP, typically have a device maximum safe condition of approximately +2 dBm, an operating condition of approximately -8 dBm, and a sensitivity of -29 dBm.
On a clear day, however, the optical input power applied to an optical detector is much greater than for a poor weather day because the optical signal is not attenuated as greatly by atmospheric conditions. As a result, the optical signal applied to the optical detector can exceed the safe operating limits of the detector and/or the first stage of the receiver saturates. When the optical input applied to a receiver exceeds the design limits for the receiver, a communication link will generally not function properly and, at an extreme, the detector may fail as a result of the large input level.
When the optical input level to an optical detector exceeds the safe operating limit for the detector, the optical receiver saturates causing the electrical output of the detector becomes distorted. To avoid the consequences of detector saturation, a conventional approach is to use an automatic gain control (AGC) amplifier stage as one of the electrical amplifier stages of the receiver. There are two possible locations in which an AGC amplifier is incorporated into circuitry of a receiver using a conventional AGC amplifier approach. The first is to integrate the AGC amplifier with the optical detector and the detector amplifier as part of a first stage. Placing an AGC amplifier at this point in the signal path results in a peaking and/or an undesirable roll-off in the frequency response as a function of the amplifier gain. The second possible location is to incorporate a monolithic AGC amplifier after an integrated detector amplifier. By placing an AGC amplifier at this point in the signal path, the large-signal handling capability of the first stage limits the optical input at the detector based on the saturation point of the detector stage.
A fallacy with using an conventional AGC circuit for avoiding the consequences of optical detector saturation is that the optical input applied to the detector is not always within the safe operating limits of the detector because the optical input power applied to the optical detector varies over a wide range based on weather conditions between the transmitter and the receiver.
What is needed is a way to enhance the optical input dynamic range of an integrated optical detector to avoid saturating the detector and for ensuring a safe operating range for the detector.